Method and Apparatus for Determining Fluid Content and Conductivity in Porous Materials
1. Field of the Invention
This invention measures the fluid content and its conductivity in porous materials such as soils containing water in which solids are dissolved.
2. Description of Prior Art
This invention uses electromagnetic fields to investigate the fluid content of porous materials. For example, the water content of soil material is determined along with the conductivity of the water. The electromagnetic fields generated by the invention are affected by the dielectric polarizability and conductivity of the surrounding media. The net dielectric constant of porous materials is altered significantly by the presence of water since it has a large intrinsic dielectric constant (80) at the frequencies used in this invention, compared to the solid porous matrix, the dielectric constant of which is typically in a range from 2 to 7. When water fills the pores in the solid matrix the average dielectric constant of the combination changes significantly. For example, the dielectric constant for a typical dry soil (average composite of soil+air in pores) is approximately 3. However, if the pores are filled with water instead of air, the average dielectric constant for the water saturated soil can be as high as 23. When water does not completely fill the pores (partial saturation) the dielectric constant lies between 3 and 23. This large contrast in dielectric properties of dry and water-saturated soil significantly affects the penetration of electromagnetic fields. The decrease in electromagnetic field penetration as the moisture content increases is the means used to evaluate the water content of the porous material. In addition, through measurements made at different frequencies the conductivity of the water can be determined from known dispersion relations for dielectrics.
There are a number of prior methods to measure the water content of soils. One of the most common is gravimetric in which a sample of soil of known volume is removed and weighed. The soil is then placed in a drying oven and again reweighed until all the water has evaporated and the dried weight no longer changes. Using the weight difference between the wet and dry soil sample and the known density of water, the volume of water loss can be calculated. This method is intrusive since it requires the removal of soil and cannot be used to monitor changes in the same place as a function of time which might result from evaporation, rain migration, or evapotranspiration from the root zone of plants. The time required to take the measurement and the accuracy of the volume measurement preclude its application to long term monitoring of water content depth profiles.
Another common technique to measure water content involves electric measurements of buffed gypsum blocks. Their conductivity changes as they absorb water from their surroundings, but their placement disturbs the soil and their response varies with soil type making them very site-specific. The conductivity of the gypsum blocks is also affected by extraneous conductivity components such as fertilizer and other soluble components in the water. The long term calibration is affected by biological growth as well as changes in conductivity in soil water.
Another technique uses fundamental interactions between nucleons to measure the presence of water in porous media. This method employs the scattering of neutrons by the hydrogen in water. The neutron source contains a radioactive isotope which requires specific safety precautions. The neutron scattering length is over 25 cm in dry soils so measurement averages over a volume approximately 25 cms in radius. This volume average does not lead to a high resolution in the water content distribution and little definition of variation in water content in regions of interest in agriculture such as the root zone of plants. The neutron scattering method also does not discriminate between hydrogen in water and organic matter or some types of clay so that certain environments limit the accuracy of the measurements.
The U.S. Pat. No. 3,944,910 of R. N. Rau teaches a technique where microwave electromagnetic fields are emitted into the surroundings and used to detect the average dielectric constant from which the water content of earth formations of interest in geophysical prospecting can be determined. The use of high frequencies (&gt;1GHz) in the microwave range means the energy does not penetrate very far into the surrounding media (approximately 1 to 2 cm). In the application described by Rau, the earth formation (consolidated rock) in the immediate vicinity of a borehole would be sampled. This is a region saturated by tiltrate from the drilling mud used to drill the borehole. The object of the microwave measurement is to determine water content and compare it with porosity measured by other techniques such as neutron scattering which does not differentiate between water and oil (hydrocarbon). The amount of pore volume not occupied by water is determined from the difference in the two measurements and is inferred to contain oil or gas. The intent of the measurement is to deal with fully saturated zones only in the immediate vicinity of the borehole to get an estimate of oil saturation and thus the use of high frequencies. The small penetration depth due to the high frequencies limits its use in sampling unaffected porous media such as soil. The apparatus has an antenna which uses a re-entrant cavity to radiate energy into the formation with a second re-entrant cavity as a receiver. The receiver is placed a significant distance from the transmitter, leading to a limited resolution of vertical changes in moisture content.
This technique of Rau's is limited to the region close to the borehole wall which it samples. This method does not yield a clear definition of the region it is sampling since there are multiple propagation paths shown in the patent. Because of the idamental size of the cavities and electronics, the borehole needed to insert the probe typically is over 15 cm in diameter, which is a significant intrusion for soil measurements. The method of evaluating either the porosity or saturation assumes that the formation is fully saturated with pore fluid. This is not independently verifiable and may not always occur, particularly if natural gas is present.
In a similar U.S. Pat. No. 4,158,165 of Coates, the formation properties are determined using a propagating microwave field around a borehole. Assuming that the infiltrated pore fluid has the same conductivity as the drilling fluid, water content is sought by measuring the microwave attenuation and phase shift. This assumption is not necessarily true when the formation contains orthogenic clays which interact with dissolved ions in the drilling fluid. To quantitatively determine the water content both Rau's and Coates' techniques require subsidiary assumptions which are not always strictly valid.
The U.S. Pat. No. 3,870,951 by Brown et. al. describes a probe which is designed to measure moisture in porous materials using a cylindrical capacitor terminating a coaxial cable. A separate cable is used to carry high frequency energy to the cylindrical capacitor. The measuring technique senses the change in the dielectric constant of the environment leading to a change in the terminating impedance in the receiving system. A DC voltage is generated which is proportional to the change in impedance of the circuit. No method is proposed to determine quantitatively the moisture content in the environment of the probe. Only relative changes are considered. The cylindrical capacitor which is the sensing unit for the moisture measurement necessarily restricts its electromagnetic field to a small region in the gap between the cylinders forming the capacitor. Therefore, it does not measure a significant lateral distance into the surrounding soil.